In a typical cellular network, also referred to as a wireless communications system or communications system, User equipments (UEs), communicate via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a mobile terminal by which a subscriber may access services offered by an operator's core network and services outside the operator's network to which the operator's radio access network and core network provide access. The user equipments may be for example communication devices such as mobile telephones, cellular telephones, smart phones, tablet computers or laptops with wireless capability. The user equipments may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server. User equipments are enabled to communicate wirelessly in the network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the communications system.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a Base Station (BS), e.g. a Radio Base Station (RBS), which in some radio access networks is also called evolved NodeB (eNB), NodeB or B node. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base station communicates over an air interface operating on radio frequencies with the user equipment(s) within range of the base station.
Standardised by the third Generation Partnership Project (3GPP), High Speed Downlink Packet Access (HSPA) supports the provision of voice services in combination with mobile broadband data services. HSPA comprises High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA) and HSPA+. HSDPA allows networks based on the Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity. In HSDPA, a new transport layer channel, High Speed-Downlink Shared CHannel (HS-DSCH), has been added to the UMTS release 5 and further specification. It is implemented by introducing three new physical layer channels: High Speed-Shared Control CHannel (HS-SCCH), uplink High Speed-Dedicated Physical Control CHannel (HS-DPCCH) and High Speed-Physical Downlink Shared CHannel (HS-PDSCH). The HS-SCCH informs the user equipment that data will be sent on the HS-DSCH, two slots ahead. The HS-DPCCH carries acknowledgment information and a current Channel Quality Indicator (CQI) of the user equipment. This value is then used by the base station to calculate how much data to send to the user equipments on the next transmission. The HS-PDSCH is the channel mapped to the above HS-DSCH transport channel that carries actual user data.
Multiple Input Multiple Output (MIMO) refers to any communications system with multiple antennas at the transmitter and receiver, and it is used to improve communication performance. The terms input and output refer to the radio channel carrying the signal, not to the devices having antennas. At the transmitter (Tx), multiple antennas may be used to mitigate the effects of fading via transmit diversity and to increase throughput via spatial division multiple access. At the receiver (Rx), multiple antennas may be used for receiver combining which provides diversity and combining gains. If multiple antennas are available at both the transmitter and receiver, then different data streams may be transmitted from each antenna with each data stream carrying different information but using the same frequency resources. For example, using two transmit antennas, one may transmit two separate data streams. At the receiver, multiple antennas are required to demodulate the data streams based on their spatial characteristics. In general, the minimum number of receiver antennas required is equal to the number of separate data streams. 4×4 MIMO, also referred to as four branch MIMO, may support up to four data streams.
Currently, a 4Tx MIMO transmissions scheme for HSDPA is discussed within 3GPP standardization. One fundamental issue is what pilot scheme to use when this feature is switched on. In HSDPA the pilot signals are used for (mainly) two things, first it is used as a phase reference when the data channel, e.g. HS-PDSCH, should be demodulated; secondly it is also used for estimating the Channel State Information (CSI) such as CQI and Precoder Matrix Indicator (PMI). The pilot signal may be a single frequency. CSI may be referred to as known channel properties of a communication link and describes how a signal propagates from the transmitter to the receiver. The CSI makes it possible to adapt transmissions to current channel conditions, which is important for achieving reliable communication with high data rates in a multi antenna communications system.
In general, most systems provide a downlink signal of a predetermined structure, referred to as the downlink pilot signal or downlink reference signal. The downlink pilot signal is transmitted from the base station with a constant power to the user equipment. The user equipment uses the received downlink pilot signal to estimate the instantaneous downlink channel conditions, which can be reported back to the base station. A Common Pllot CHannel (CPICH) is a channelization code used for transmission of the pilot signal. The common pilot channel comprises known data and is used as a reference for downlink channel estimation by all user equipments in the cell served by the base station. Unlike the downlink, where the common pilot signal is used, uplink transmissions originate from different locations. Therefore, a common pilot signal cannot be used in uplink transmissions. In uplink transmission, each user equipment must have a separate dedicated pilot signal. The dedicated pilot signal is carried on the Dedicated Physical Control CHannel (DPCCH)
For a four branch MIMO system, pilot signals are needed for two main functionalities; channel state information estimation through channel sounding where the channel rank, CQI and Precoding Channel Indicator (PCI) are estimated and channel estimation for demodulation purposes. The wireless device recommends the channel rank information to the base station. In order to support 4Tx MIMO transmissions new pilot signals need to be defined. For four branch MIMO the following two different approaches are possible:                Common pilot signals for both CSI estimation and channel estimation.        Common pilot signals for CSI estimation and dedicated pilot signals for channel estimation.        
Either the common pilot signal scheme of today is expanded to support four transmission antennas (4Tx). In this case the same pilot signals may be used for both demodulation purposes as well as for CSI estimation. Alternatively, dedicated pilot signals may be defined for demodulation of the data channel. In addition to these, some sparse and/or low power common pilot signals are needed for CSI estimation. These common pilot signals are needed since dedicated pilots in general may not be used for CSI estimation. This since they only exists when data to a certain user equipment is scheduled, and CSI estimation should be done by all user equipments in the cell. Also, if the dedicated pilot signals are precoded, the user equipment may not use this for precoder estimation since they in general do not span the total channel subspace. The exception is when a full rank transmission occurs to a user equipment.
The two pilot signal schemes have their pros and cons, but in this disclosure the focus is on the case where only common pilot signals are defined for both demodulation and CSI estimation purposes.
The common pilot signal approach for CSI has been investigated, and the channel estimation option as well as an option with dedicated pilot signals along with common pilot signal solution used for estimation of the channel for CSI estimation. Code multiplexed dedicated pilot signals rather than time division multiplexed pilot signals should be considered in the evaluation for data demodulation.
In the following, the impact of introducing common pilot signals only solution is described, i.e. common pilot signals are used for channel estimation and used for both CQI estimation and data demodulation.
The main problem occurring when the number of pilot signals is expanded from one (or two) to four is that there is a negative impact on legacy user equipments. A legacy user equipment may be a user equipment which is old and which continues to be used, typically because it still functions for the user equipment's needs, even though newer technology or more efficient methods are available The main impact on system performance stem from the high rank interference that is spread in the system. It is well known that a receiver with N Rx antennas may suppress interference from N−1 sources, where N is a positive integer. Hence, a typical user equipment receiver with two Rx antennas may only suppress one interfering signal. When 2Tx MIMO was introduced this was seen as a main obstacle, and the solution was to reduce the power of the second pilot signal sent from the second transmit antenna. Normally, the second pilot signal is transmitted with 3 dB, i.e. half the power, of the primary common pilot signal. This would minimize the impact on legacy user equipments while keeping the quality of the channel estimates of the second channel on an acceptable level. When going to 4Tx transmissions this may be generalized so that the power of the third and fourth pilot signal is reduced even further to minimize the impact on legacy user equipments. However lowering the pilot power also means worse channel estimation performance and hence lower throughput.
In FIG. 1 the impact of additional common pilots on legacy user equipments is depicted. In FIG. 1, the site-to-site distance is 500 m and the user equipment speed is 3 km/h. The x-axis of FIG. 1 represents the cell throughput measured in Mbit/s and the y-axis represents the mean user equipment throughput measured in Mbits/s. The circled line illustrates a R99 user equipment implementing 4Tx and where the second, third and fourth pilot power is −13 dB. The starred line illustrates a R99 user equipment implementing 2Tx and where the second pilot signal is −13 dB. The triangle line illustrates a R99 user equipment implementing 1Tx. It is seen that adding one additional pilot to the 1Tx scenario, i.e. 2Tx, has some negative impact on a R99 user equipment even if the power of the second pilot is reduced by 3 dB. However when adding 3 additional pilots, i.e. 4Tx, the throughput is reduced by 50% even if the pilot power is reduced.
FIG. 2 shows the system level impact on a Release HSDPA system with two transmit antennas and two receive antennas due to additional pilot power for Release-7 MIMO user equipments. The x-axis of FIG. 2 represents the number of user equipments per sector. The y-axis represents the sector throughput measured in Mbps. The squared line represents the third and fourth pilot power equals −13 dB. The circled line represents 2×2 MIMO only. It may be observed from FIG. 2 that due to additional pilot power there is a loss in throughput due to additional interference. The loss is significant at higher load. From this it is clear that in order to keep the impact on a reasonable level, the pilot power need to be reduced much more than 3 dB. This will, on the other hand have a negative impact on the MIMO user equipment since the channel estimates for the additional antennas will be very poor. This would mean that the throughput of this user equipment will be severely reduced.